[sdiy] Funky flip-flops

[Harry Bissell Jr]

You can interface 4000 series with TTL but it has
some problems.  The 4000 series threshold is about 1/2
Vdd (or 2.5V).   TTL levels are less than .8V for
low... and greater than 2.4V for high.  The 2.4V and
2.5V are close enough that it might be marginal.

There are some good 4000 series datasheets (app notes)
that deal with interfacing CMOS. In general TTL can
drive CMOS, but might need a pullup resistor. CMOS
may not have enough current drive to reach the low TTL
threshold... requiring a buffer.

74HCT COMS is 'ttl' compatible for threshold... but
still lacks the big current needed to drive TTL.

H^) harry


--- Karl Ekdahl <_nial_ at yahoo.com> wrote:

> All in all i counted on 28 TTLs for three boards,
> about 8 on the first one and the rest on the second
> one. Anyhow i desoldered some of them yesterday, to
> make room for CMOS instead. I noticed that i did
> have
> 40xx replacements for most parts, and a few LS
> replacements will be made as well. Roughly counting
> power dissipation i'm going from 230mW to 12mW.....
> 
> Oh btw, is it safe to mix 74LSxx with 40xx? I'm
> still
> going to run it on 5v in that case of course....
> 
> Karl
> 
> 
>  --- harrybissell <harrybissell at prodigy.net> skrev: 
> > If you have an output shorted to an output... the
> > internal resistance
> > of the devices is in the circuit. It will get hot
> > but might not draw the
> > huge
> > current you are describing...  just more than
> > normal.
> > 
> > How many chips on each board ?
> > 
> > H^) harry
> > 
> > Karl Ekdahl wrote:
> > 
> > > I looked for shorts and outputs going directly
> to
> > VCC
> > > or GND but didn't find anything. I thought that
> if
> > any
> > > of the two boards i'm making (it will be 3
> boards
> > > eventually) is having some kind of short-circuit
> > or
> > > likewise, it should draw a tremendous amount of
> > power
> > > so i measuerd the mA usage of each board.
> > Apparently
> > > the 3 x 2 digit LED displays draw 30mA together,
> > the
> > > first board around 220mA and the second 210mA,
> > does
> > > this sounds reasonable? If so, i guess this baby
> > will
> > > draw around 1A when i'm done, thus i can totally
> > > forget of making this one battery operated as my
> > first
> > > thought was. Maybe i should just drop this and
> do
> > it
> > > in PIC, though then i won't get rid of any 74
> > chips.
> > >
> > > Karl
> > >
> > >  --- harrybissell <harrybissell at prodigy.net>
> > skrev:
> > > > Whenever something happens with a scope it is
> a
> > > > clue.
> > > > I'd say there is no way the scope should knock
> > out
> > > > your
> > > > power supply.  I'd suspect that you have the
> > scope
> > > > connected
> > > > to ground (third pin if you're USA - are you
> > ?)...
> > > > and when you
> > > > hook up the scope ground you are making an
> > unwanted
> > > > connection ?
> > > > Or maybe the scope is mistakenly a 50ohm input
> > (rare
> > > > but possible) ?
> > > >
> > > > LEDs can eat amazing current... you can
> > calculate
> > > > from the series
> > > > resistor, the power supply voltage and figure
> a
> > 1.5V
> > > > drop in the
> > > > LED. Three seven segment displays could be
> more
> > than
> > > > 200mA
> > > > alone.
> > > >
> > > > I'd look into that scope thing carefully...
> > > >
> > > > H^) harry
> > > >
> > > > Karl Ekdahl wrote:
> > > >
> > > > > So, i did add the 0.1uF decoupling caps and
> > fired
> > > > the
> > > > > circuit, unfortuanley it behaved even more
> > strange
> > > > > than before so i tried plugging in my scope
> > > > looking
> > > > > for noise in the powerlines. Even more
> > unfortunate
> > > > is
> > > > > the fact that every time i plug in my scope
> > (an
> > > > old
> > > > > Tek scope) the fuse in my powersupply
> brakes.
> > > > > Apparently, the 74xxx does draw a lot of
> > current,
> > > > but
> > > > > is it really so that a couple of 15 74xx's +
> 3
> > LED
> > > > > displays should blow a 500mA fuse? It's
> > becoming
> > > > clear
> > > > > to me that i have NO idea whatsoever how
> much
> > > > current
> > > > > things actually draw. And by the way, yes,
> the
> > > > 74xxx's
> > > > > are free - i've got shitloads and i want to
> > get
> > > > rid of
> > > > > them so i decided to build an
> overcomplicated
> > > > > drummachine sequencer. I've also been
> thinking
> > of
> > > > > building a *huge* pocket calculator only
> using
> > > > simple
> > > > > logic 74xxxx's.
> > > > >
> > > > > Karl
> > > > >
> > > > >  --- harrybissell <harrybissell at prodigy.net>
> > > > skrev:
> > > > > > Yes Jay...
> > > > > >
> > > > > > I'd like to add... decoupling caps. TTL is
> > > > > > desperately in need of good power supply
> > > > > > decoupling.   I'd suggest a .1uF cap
> across
> > > > EVERY
> > > > > > single TTL chip.   I follow this rule even
> > > > > > with 4000 series CMOS... which could use
> > > > probably
> > > > > > half that amount.
> > > > > >
> > > > > > About the RC on the ... clock line ??   I
> > would
> > > > not
> > > > > > use a filter on the clock line... TTL
> needs
> > > > > > a good fast pulse to trigger well...
> > although I
> > > > have
> > > > > > seen some times where a tiny tiny RC
> > > > > > helps in stopping a bounce (and
> mistrigger).
> > > > Better
> > > > > > to fix the real problem than try to mask
> > > > > > it.
> > > > > >
> > > > > > A modest proposal... switch to 4000 series
> > CMOS.
> > > >  It
> > > > > > is way, way, way easier to use than TTL,
> > > > > > uses a fraction of the power...
> > > > > >
> > > > > > ...unless the TTL stuff was free, of
> > course...
> > > > > >
> > > > > > H^) harry
> > > > > >
> > > > > > jays at aracnet.com wrote:
> > > > > >
> > > > > > > Reset sounds funky.
> > > > > > >
> > > > > > > 1) Holding the reset lines low has the
> FF
> > in a
> > > > > > constant state of reset. They are active
> low
> > and
> > > > > > should be tied high (reset to +V) with a
> > 1K-3K
> > > > ish
> > > > > > resistor. Or better yet put a cap in there
> 
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